1. Field of the Invention
The present invention relates to positioning systems, and more particularly to aiding positioning systems.
2. Description of Related Art
Satellite positioning systems, such as Global Navigation Satellite Systems (“GNSS”) and Global Positioning Systems (GPS), have fostered the development of a wide variety of applications, ranging from navigation systems in mobile units (e.g. automobiles, mobile telephones, etc.), to the E911 emergency system, which requires the ability to locate callers on mobile telephones. The growth of applications that use positioning systems, both in terms of number and sophistication, has placed increasing demands on satellite positioning system (“SPS”) performance.
One problem with satellite positioning systems that continues to draw attention is that positioning capabilities become increasingly limited as the lines of sight to satellites are reduced. In urban areas with large buildings, signals reflect off the buildings creating multiple signals taking different paths to SPS receivers. The SPS receivers may not be able to communicate with enough SPS satellites to determine a location, or at least to do so with enough precision. In large buildings, there may be no lines of sight to SPS satellites making it very difficult for positioning receivers to precisely determine their location.
Another problem relates to how a SPS receiver “finds” itself once the SPS receiver has been left unused and then moved to a different location. SPS receivers constantly update their location during operation. When a SPS receiver is not used, then moved to a different location, it must “wake up” and determine its new location. When the SPS receiver is restarted, it begins the process of determining its new location by searching for available SPS satellites.
Each SPS satellite generates and broadcasts its own 1023-bit Pseudo Random Noise (PRN) codes, these are pseudo random sequences that a SPS receiver knows about and tries to match by generating that same signal in order to identify a particular satellite. SPS receivers also encode the time a signal is being transmitted. The SPS receiver searches for this signal in terms of time (the signal is transmitted at 1023 Mbits/sec) and frequency (added Doppler effect from the satellite movement). Each satellite sends its PRC (Pseudo Random Code), position and the current time 50 times per second. After locking up a signal, the SPS receiver receives this data and calculates the time difference from when the signal was sent to the Time of Arrival (TOA). Included among the data received at this time are the almanac and ephemeris.
On a “cold start” (i.e. the SPS receiver lacks any ephemeris data) the SPS receiver knows where to search for a satellite in the sky based on the almanac data available and will try to obtain ephemeris data from each visible satellite. Satellites broadcast their ephemeris data every 30 seconds. If the SPS receiver is blocked while trying to obtain the data, it will have to start over in the next cycle. On a “warm start” some ephemeris is already available and the receiver can almost “guess” its position and in a “hot start” the receiver has almost all of it readily available. This whole process can translate in minutes or seconds of wait time depending in the kind and quality of data available for a SPS receiver to obtain a fix.
Mechanisms for assisting SPS receivers in lowering their time to first fix (“TTFF”) have been implemented. For GPS systems, these mechanisms include “Assisted GPS” and “Aided GPS.” Aided GPS is generally understood to involve ephemeris or almanac data aiding. Assisted GPS uses data from a wireless network infrastructure, sometimes down to location information itself based on the Cell ID from a mobile phone.
In one example of an aided GPS system, a gateway to a telecommunications system may be used to maintain continuously updated ephemeris and almanac data for nearby GPS satellites. In a cold start, a GPS receiver may request ephemeris data from the gateway via a request to the telecommunications system. Although such an aided GPS system may improve TTFFs in some implementations, the need to make a request still takes time that may add significantly to the TTFF.
There is a need for aided positioning systems that further reduce the time for GPS receivers to acquire ephemeris data.